checkpoints are occurring too frequently

I have a lot of entries like this in the log file

2011-11-17 02:02:46 PYST LOG:  checkpoints are occurring too frequently (13 seconds apart)

2011-11-17 02:02:46 PYST HINT:  Consider increasing the configuration parameter "checkpoint_segments".

No, checkpoint parameters in postgres.conf are:

checkpoint_segments = 32                          # in logfile segments, min 1, 16MB each

checkpoint_timeout = 10min                      # range 30s-1h

checkpoint_completion_target = 0.9      # checkpoint target duration, 0.0 - 1.0

What should be a correct value for checkpoint_segments to avoid excessive checkpoint events?